1. Field of the Invention
The present invention relates to an ink set excellent in discoloration balance in image preservation and also relaters to an ink jet recording method.
2. Description of the Related Art
With the spread of the computer in recent years, ink jet printers are widely used not only in offices but also in homes for printing on papers, films and cloths.
As ink jet recording methods, a method of ejecting ink droplets by applying pressure by piezo-elements, a method of ejecting ink droplets by generating bubbles in ink by heat, a method of using ultrasonic waves, and a method of suction-ejecting ink droplets by electrostatic force. As the ink compositions for the ink jet recording methods, water ink, oil ink and solid ink (a melting type) are used. Of these inks, water ink is mainly used for easiness of manufacture and handling, odor and safety.
Colorants used in inks for ink jet recording are required to have high solubility in a solvent, to be capable of high density recording, to be good in hue, to be excellent in fastness to light, heat, air, water and chemicals, to have good fixing property to an image-receiving material and to hardly blot, to be excellent in preservation property as ink, to be free of toxicity, to be high in purity, and to be available inexpensively. However, it is extremely difficult to search for colorants that satisfy these requirements in high level.
Various dyes and pigments have been already proposed for ink jet recording and practically used but colorants satisfying all of these requirements have not been discovered yet. It is difficult for conventionally well-known dyes and pigments having Color Index Nos. (C.I.) to reconcile the hue and fastness required of the inks for ink jet recording.
As the dyes that improve the fastness, azo dyes derived from aromatic amine and 5-membered heterocyclic amine are proposed in JP-A-55-161856. However, since these dyes have undesirable hues in yellow and cyan regions, there is a problem of the deterioration of color reproduction.
Inks for ink jet recording aiming at the compatibility of hue and light fastness are proposed in JP-A-61-36362 and JP-A-2-212566. However, the dyes used in these patents are insufficient in water-solubility when used as water-soluble inks. In addition, when these inks are used as water-soluble inks for ink jet recording, there also arises a problem in the fastness to humidity and heat.
As the means for solving these problems, the compounds and ink compositions disclosed in JP-T-11-504958 (The term “JP-T” as used herein means a published Japanese translation of a PCT patent application.) are proposed. In addition, ink for ink jet recording using a pyrazolyl aniline azo dye for improving hue and fastness is disclosed in JP-A-2003-231850. However, these inks for ink jet recording are not sufficient both in color reproduction and the fastness of outputted images.
Further, it was found that the preservation property of the image recorded on special purpose glossy paper of photographic quality for ink jet recording and stuck in a room was conspicuously bad in some cases. The present inventors conceive that this phenomenon must be due to any oxidizing gas in the air, e.g., ozone. Although the phenomenon is difficult to occur by shielding the flow of air by, e.g., putting the image in a frame with glass, by which the use condition is limited.
The phenomenon is especially conspicuous with special purpose glossy paper of photographic quality for ink jet recording, therefore, this is a great problem to the present ink jet recording where the photographic quality is an important characteristic.
The present inventors have examined dyes having good hue and fastness and advanced the development of excellent colorants for ink jet recording.
In the ink set comprising a plurality of inks for ink jet recording in combination, a technique of using two or more kinds of inks having the same hue and different densities has been developed for further increasing image quality.
For instance, methods of improving discoloration balance by using a set of dyes fast to discoloration are disclosed in JP-A-2002-371213 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”). However, in these methods, the discussion on the compositions of light ink and dark ink is insufficient, and the methods simply varied the concentrations of dyes. In light ink, the ratio of a water-soluble organic solvent to a dye in the ink is high, so that light ink is inferior to dark ink in fastness, as a result the color balance of the image comes to be lost.